This is a randomized, double-blind, parallel-group, placebo-controlled, multi-center trial to evaluate the safety and efficacy of Leustatin (Cladribine) in subjects with chronic progressive multiple sclerosis. There will be three patient groups in the trial: (1) a placebo group: (2) a high dose treatment group, and (3) a low dose treatment group. The placebo group is necessary to establish the magnitude of changes in clinical endpoints that may occur in the absence of active treatment.